1. Field of the Invention
This invention relates generally to a spiral arm antenna and, more particularly, to a wideband, multi-mode, center-fed/end-fed, spiral arm antenna that simultaneously senses both right-hand circularly polarized and left-hand circularly polarized signals.
2. Discussion of the Related Art
Tactical military aircraft operating in a warfare scenario typically radar and communications signals. These signals may be low frequency UHF and VHF signals, radar frequency signals, or high frequency signals (0.3-18 GHz). These signals may be cross-polarized signals that are either right-hand circularly polarized (RHCP) or left-hand circularly polarized (LHCP) or a combination of the two. The sense of the polarization defines the rotation of the signal as it propagates.
Aircraft are generally equipped with signal sensing systems that sense the radar and communications signals, and then determine angle of arrival (AoA) and calculate the direction of the signals. This allows the pilot of the aircraft to take evasive or other actions. To be effective in modern warfare, these sensing systems must employ an antenna system that is able to simultaneously detect both RHCP and LHCP signals in the frequency band of interest.
Multiple arm spiral antennas are known in the art for their ability to sense RHCP and LHCP signals. The known multiple arm spiral antenna systems typically include a plurality of spiral antenna arms spiraling out from a common central location. The antenna feed for each separate arm is generally connected to the end of the arm at the common central location. U.S. Pat. No. 3,681,772 discloses a spiral antenna that includes multiple spiral arms radiating out from a common center, where the arms are connected to the antenna feed only at the central location. Patent, '772 generates the counter rotating modes by reflecting currents from impedance discontinuities in the arms. This spiral antenna is sensitive to both RHCP and LHCP signals. Additionally, U.S. Pat. No. 4,658,262 also discloses a dual polarized sinuous antenna that includes a plurality of spiral antenna elements extending from a common central location. The sinuous antenna disclosed in this patent is also only fed at this common central location of the arms.
Modern military aircraft are low-observable aircraft that have small radar signatures. To maintain this low-observability, any antenna system mounted on the aircraft must conform with the aircraft structure and not increase its radar cross-section (RCS). The conductive material in the antenna, however, adds to the RCS. Sharp edges of the antenna elements also provide a significant increase in the RCS at certain frequencies. Both of the spiral arm antennas disclosed in the '772 and '262 patents have significant RCS because the arm elements include sharp edges and transitions that add to the radar visibility. These transitions of the arm elements in the '772 and '262 patents are important to allow the antenna to sense both RHCP and LHCP signals when only being fed at the ends of the arms radiating from the antenna center.
Additionally, the antenna system for providing AoA estimations should detect higher order RHCP and LHCP modes to provide a higher relative phase rate to reduce the ambiguities of the AoA estimations, and make it more accurate. The more arms that are available, the more modes generated. Because higher order modes provide greater AoA accuracy, it is desirable to provide more modes without providing more arms so as to not increase the RCS. Less arms also decreases system fabrication costs and antenna system hardware.
What is needed is a multi-mode spiral arm antenna that simultaneously provides both RHCP and LHCP sensitivity, and provides higher order mode generation for increased AoA accuracy and a smaller RCS. It is therefore an object of the present invention to provide such an antenna.